The present invention relates to providing gate drive to switching devices. More specifically, the present invention provides a flyback energy storage technique to provide a current pulse with a fast rise time for driving fast semiconductor switching devices.
Certain types of high power semiconductor switching devices require gate drive pulses with very fast rise times for proper operation. For example, a high power pulse thyristor or a silicon controlled rectifier (SCR) may require a 30 amp pulse with a 200 nanosecond rise time. One technique for driving a stack of such devices is described in commonly assigned, copending U.S. patent application Ser. No. 09/007,574 for COMPACT SOLID STATE KLYSTRON POWER SUPPLY filed on Jan. 15, 1998, the entirety of which is incorporated herein by reference for all purposes. In that application, each of the gates of an SCR stack are driven by a corresponding secondary winding of a multiple secondary, single primary pulse transformer.
The "hard switch" approach favored by many in the industry uses a high voltage source which employs a specially crafted ultra-low-inductance storage capacitor and charging supply. When a gate pulse is required, the high voltage supply is switched onto the primary of the gate drive pulse transformer. Unfortunately, the inductance of such a pulse transformer circuit, while relatively small, is typically large enough to necessitate a very high voltage power source on the primary winding which also must be capable of sourcing a lot of current. That is, the bulk inductance of the wires leading to the primary winding from the power source and the combined leakage inductances of the multiple secondary windings reflected back through to the primary are such that a very large amplitude, high current drive must be provided to overcome the effective primary inductance and generate the required amplitude and rise time, i.e., di/dt, on each of the secondary windings.
An example of the power source necessary for driving a stack of pulse thyristors which require 30 amps in 200 ns should be illustrative. Assuming a typical primary lead inductance of 2 .mu.H and a 1:5 primary-to-secondary turns ratio, a 3000 volt supply able to source 150 amps must be used. Where the number of device gates being driven (and therefore the number of secondary windings) is high, leakage inductances must be taken into account, making the voltage requirement significantly higher.
The disadvantages of such an implementation are well known to those of skill in the art. High voltage power supplies and their associated circuitry require special design considerations, present safety issues, and are typically less reliable and more expensive than their lower voltage counterparts. For example, pulse transformers in such high power applications must be robust, fast, and efficient, making them both bulky and expensive. In addition, stray reactances in such designs contribute to the generation of formidable transients which must be suppressed.
It is therefore desirable to provide improved gate drive circuitry for fast semiconductor devices which employs a relatively low voltage power supply and which is simpler, more reliable, and less expensive than current solutions.